In conventionally known adaptive modulation communication, a state of a propagation path is estimated to use as a criterion in adaptively selecting modulation parameters (MCS: Modulation and Coding Scheme) comprised of a plurality of modulation schemes, coding rates in error correction, or combinations thereof.
Generally, the state of the propagation path is estimated as a value such as Carrier to Noise Ratio (CNR) from a reception state in a receiver, using a preamble which is a known signal and inserted in a radio frame in a transmitter (Non-patent Document 1). Further, in sub-carrier adaptive modulation communication in a multicarrier transmission scheme, the propagation path state is estimated for each subcarrier, and a modulation parameter is selected for each subcarrier.
FIG. 24 is a diagram showing an apparatus configuration example of a transmitter in a conventional Orthogonal Frequency Division Multiplexing (OFDM) subcarrier adaptive modulation system. A propagation path estimating circuit 91 receives information such as, for example, CNR, delay profile or the like of a reception signal transmitted as feedback from a receiver, estimates the propagation path state from the information of the reception signal, and outputs the estimation result to a modulation parameter selecting circuit 92. The modulation parameter selecting circuit (uniquely) selects a modulation parameter from the input estimation result of the propagation path state, and outputs information of the selected modulation parameter to a subcarrier adaptive modulation and coding circuit 93.
The subcarrier adaptive modulation and coding circuit 93 receives transmission data, and performs adaptive modulation on the transmission data for each subcarrier corresponding to the selected modulation parameter. An IFFT circuit 94 performs inverse Fourier transform on an output of the subcarrier adaptive modulation and coding circuit 93 to transform into a digital OFDM signal, and outputs the signal to a D/A conversion circuit 95. The D/A conversion circuit 95 coverts the digital OFDM signal into an analog signal.
Further, as another conventionally known adaptive modulation communication, there is a scheme for predicting a state of a propagation path using time extrapolation of the propagation path, and based on the predicted state of the propagation path, adaptively selecting a modulation scheme (Non-patent Document 2). This time extrapolation of the propagation path is one of methods for predicting a propagation path state at the time a next communication is performed, from a past estimation value of the propagation path state and a latest estimation value of the propagation path state. FIG. 25 is a graph showing an example of time extrapolation of the propagation path state by linear approximation. A prediction value (propagation path time extrapolation value) pn+1 of the propagation path state at time tn+1 a communication is next performed is obtained from propagation path estimation value pn−1 at past time tn−1 and propagation path estimation value pn at latest time tn, using Equation (1).
                    [                  Eq          .                                          ⁢          1                ]                                                                                      ⁢                              p                          n              +              1                                =                                    p              n                        +                                          (                                                      p                    n                                    -                                      p                                          n                      -                      1                                                                      )                            ⁢                                                                    t                                          n                      +                      1                                                        -                                      t                    n                                                                                        t                    n                                    -                                      t                                          n                      -                      1                                                                                                                              (        1        )            
FIG. 26 is a diagram showing an apparatus configuration example in a conventional OFDM subcarrier adaptive modulation system using time extrapolation of the propagation path. A propagation path estimating circuit 260 receives information such as, for example, CNR, delay profile or the like of a reception signal transmitted as feedback from a receiver, estimates the propagation path state from the information of the reception signal, and outputs the estimation result to a storage circuit 261 and time extrapolation circuit 262. The storage circuit 261 stores the input propagation path estimation result, and outputs a past propagation path estimation result to the time extrapolation circuit 262. The time extrapolation circuit 262 performs time extrapolation of the propagation path, based on the latest propagation path estimation result input from the propagation path estimating circuit 260 and the past propagation path estimation result input from the storage circuit 261, and outputs a propagation path time extrapolation value that is a prediction value of the propagation path state to a modulation parameter selecting circuit 263.
The modulation parameter selecting circuit 263 (uniquely) selects a modulation parameter from the input propagation path time extrapolation value, and outputs information of the selected modulation parameter to a subcarrier adaptive modulation and coding circuit 264. The subcarrier adaptive modulation and coding circuit 264 receives transmission data, and performs adaptive modulation on the transmission data for each subcarrier corresponding to the selected modulation parameter. An IFFT circuit 265 performs inverse Fourier transform on an output of the subcarrier adaptive modulation and coding circuit 264 to transform into a digital OFDM signal, and outputs the signal to a D/A conversion circuit 266. The D/A conversion circuit 266 coverts the digital OFDM signal into an analog signal.    Non-patent Document 1: Suzuki et al. “An Estimation Scheme of Propagation Path Characteristics for an Adaptive Modulation System” IEICE Technical Report, September, 1994, RCS94-65, p. 37-42    Non-patent Document 2: Yoshiki et al. “Performance of the OFDM based Adaptive Modulation System with a Multilevel Transmit Power Control Scheme using Delay Profile Extrapolation Technique for High Mobility Terminals”, IEICE Technical Report, May, 2002, RCS2002-58, p. 41-46